In the field of helicopter control systems, among other things linear actuator units are used, by means of which control inputs from the pilot can be transmitted by operating elements to control components of the helicopter. For this, in modern controls electric systems are used increasingly, by which these control signals are converted electro-mechanically into linear control movements. These electro-mechanical linear actuator units are usually made as redundant components so that even in the event that an individual component should fail, the flight can continue safely and the helicopter can be landed.
From US 2005/0168084 A1 an electro-mechanical linear actuator unit is known, in which two electric drives are accommodated in an actuator housing, each of which independently of one another sets into rotation a plurality of screw-threads of a respectively associated spindle drive. In this case, by virtue of the drive and rotation of the threads of the first spindle drive the actuator housing is displaced in a linear direction relative to a surrounding housing, while by actuating the threads of the second spindle drive by means of the electric drive associated therewith a control rod can be displaced linearly. Thus, the electric drives are located in the actuator housing in such manner that relative linear adjustment of the control rod relative to the housing can be produced, on the one hand, as the sum of the adjusting movements and adjustment speeds of the two spindle drives, but also, on the other hand, especially if one of the spindle drives should fail, by means of a single spindle drive.
By summing together the adjustment paths covered by and the speeds of the individual spindle drives, relative adjustments of the control rod to be produced in normal operation can be distributed uniformly between the two spindle drives and electric drives. This allows the adjustment speeds and accelerations to be produced by the individual electric drives to be halved, which ultimately also enables the individual electric drives to be made smaller and hence lighter.
Moreover, it is generally known that an electro-mechanical linear actuator unit of such type comprises a control device which actuates the two electric drives in accordance with control specifications from a pilot.
However, an electro-mechanical linear actuator of the prior art has the disadvantage that when high external axial forces are acting on the control rod, the electric drives have to be continually electrically acted upon by the control device in order to maintain a set linear position. But in the event that an electric drive has failed, this can result in the occurrence of undesired relative adjustments of the control rod, or in the undesired compensation by the spindle drive of the faulty drive, of actuation movements produced by the still functional electric drive.